Both plants and animals have evolved ways to prevent interbreeding between species. In other words, each species is reproductively isolated from other species because of reproductive barriers that prevent hybridization. The focus of this research is to understand the nature of reproductive barriers between species within the genus Solanum, which includes two important crop species, potato and tomato. We have determined, the timing of reproductive barrier formation and site of barrier action in inter-species crosses, and male and female genes involved in forming reproductive barriers have been identified. This information is being used to pursue the detailed molecular mechanisms that constitute inter-species recognition and rejection during mating attempts. Prior research has identified a population of S. habrochaites (a wild tomato species) with incipient reproductive barriers that isolate it from other populations. This system will now be developed as a powerful model for answering fundamental questions about how new species evolve. Sstudies are being conducted using tomato because it is an excellent model system for genetic and genomic studies. Research on reproductive barriers will also be expanded into potato, an increasingly important crop worldwide.